James M. Palmer

Is Sunglint Observable in the Thermal Infrared

The short answer is yes. Orbiting spacecraft like GOES, AVHRR, ASTER, MODIS, and ATSR all have spectral observation windows in the thermal infrared. They all have to deal with sunglint at least some of the time. It is not just a minor nuisance, but a major effect that saturates their sensors. A saturated sensor gives no indication of the actual magnitude; all information from that direction is lost. The directional characteristics of sunglint have been adequately treated by Maurice Cox. This report deals with the radiometric aspects, the magnitude of the sunglint in comparison with the magnitude of normal target thermal emission.

Propagation of Optical Radiation

A ray is represented by a vector: a straight line indicating the magnitude and direction of propagation. A wavefront is a notional surface locally normal to a ray. Thus, a wavefront could be a plane (all rays parallel, as if from infinity) or a curved surface (indicating diverging rays, as if emanating from a point). Figure 2.1 illustrates rays and wavefronts for both cases. A beam can be defined by two separated elements of area as shown in Fig. 2.2. It is thus the locus of possible rays that pass through the two areas separated by distance d. As dA1 and dA2 approach zero, the beam approaches a single ray. Since there is a small, but nevertheless finite, cross-sectional area associated with a beam, it is capable of carrying power without the flux density (power per unit area) approaching infinity.